This invention relates to mooring systems for tensioned leg platforms and in particular to a bearing located within a mooring tunnel to absorb tension leg strains.
After an offshore well is drilled from a floating platform, it may be desirable to produce the well to a later-installed tensioned leg platform. These tensioned leg platforms, while supported by the buoyancy of the water, are not freely floating but are tied back to a plurality of anchors on the seabed.
An anchor template is affixed to the seabed, and an anchor connector with a flexible element connects the lower end of a tension line to an anchor template. The tension line which is formed of a length of heavy wall tubular elements connected by threaded joints is connected to an upper flexible element located within the lower hull structure of the tensioned leg platform. A tension load base block which carries the reaction loads between the tension line and the structural framing of the platform is located above the water level, and a tension adjustment assembly is located at the support elevation, with means for sensing the load also included.
A lower spherical flex element is required at the anchor connector. An upper spherical flex element is also located at the tensioned leg platform. These permit the platform to translate horizontally due to the forces of wind, waves, and current. These flexible elements allow the tension leg to deflect locally at the anchor template and at the lower surface of the platform hull. In theory, the length of the tension leg is held constant by the threaded nut in the tension adjustment and support assembly. However, large ocean swells cause the waterline to move vertically relative to the platform as it is anchored by the tension line, thus changing the displacement and in turn the net buoyant force of the platform. This change in buoyant force causes a variation in tension. For instance with a nominal tension of 1,000 tons, the cyclic variation due to waves in a 100-year storm causes the tension to vary in the range of plus and minus 400 tons.
This variation in tension causes a resultant variation in the strain of the tension line. As the strain varies in the tension line, the upper flexible element is forced to move vertically relative to the tensioned leg platform structure as a function of the strain variation or stretch in the portion of the tension leg between the support location on the platform and the flexible element. This distance is in the order of 100 feet, and the movement is typically in the order of 11/2 inches.
This relative vertical motion is concurrent with large horizontal offsets of the platform relative to the anchor template which causes the tension line to assume an angle relative to the vertical. The horizontal component of the tension in the line is then reacted as a side load against the lower hull structure, or mooring tunnel, of the platform through the upper flexible element. Side loads due to this may be as great as 100 tons. Thus, the upper flexible element is forced to oscillate vertically relative to the platform as it is simultaneously loaded horizontally against the interior wall of the platform mooring tunnel. This creates an extremely high wear situation at this location where components are extremely difficult to replace.